1. Field of the Invention
This invention relates to a process for producing polyethylene materials of improved mechanical strength and elastic modulus characteristics by a specified mode of molding.
2. Description of the Prior Art
An ethylene polymer of an extremely high molecular weight, commonly called ultrahigh molecular weight polyethylene, is well known to have superior resistance to impact and to abrasion and peculiar self-lubricity. This type of polyethylene enjoys high reputation as an engineering plastic material and finds extensive application to hoppers, silos, gears, linings and the like for use in various sectors of industry ranging from food processing, civil engineering, chemistry, agriculture and mining to sports and leisure supplies
Polyethylene of ultrahigh molecular weight is literally high in melting viscosity and hence difficult to extrude or stretch under usual molding conditions. This is interpreted to mean that the benefits of such polymer cannot be attained to a full extent.
In Japanese Patent Laid-Open Publication No. 56-15408 it has been proposed to stretch a certain gel derived from a decalin dope of ultrahigh molecular weight polyethylene, thereby forming highly strong, elastic fibers. This dope however can only be formed with a limited polymer concentration, say 3% by weight of a polyethylene having weight-average molecular weight of 1.5.times.10.sup.6 or 1% by weight of a polyethylene having a similar molecular weight of 4.times.10.sup.6. Such prior stretching has a drawback in that it requires much solvent and much care in dissolving the polymer in the solvent, leading to lesser economy and inconvenient handling.
Alternatively, polyethylenes of ultrahigh molecular weights have been oriented to a high degree as disclosed for instance in Japanese Patent Laid-Open Publication No. 59-187614, No. 60-15120 and No. 60-97836 and Preprints of the Society of High Polymers, Japan, vol. 34, p. 873 (1985). In such prior modes of molding a dilute solution of that polymer in xylene, decalin, kerosine or a similar solvent is cooled or crystallized isothermally to form a single crystal mat which is thereafter extruded or stretched in a solid phase. This is still uneconomical with much solvent necessary for mat formation.
Furthermore, Japanese Patent Laid-Open Publication No. 63-41512and No. 63-66207 teaches stretching ultrahigh molecular weight polyethylene without resorting to dissolution or melting. This leaves the magnitude of orientation unsolved.